ISTANBUL (AP) — A Turkish court on Monday convicted five mayors of municipalities run by the country’s main opposition party on corruption charges in a trial that critics have denounced as politically motivated. The case before an Istanbul court included allegations of bribery, bid-rigging and fraud in relation to municipal contracts. Critics of the government of Turkish President Recep Tayyip Erdogan say the trial targeted the opposition Republican People’s Party, or CHP, and that the prosecution relied on secret witnesses and defendants implicating other defendants in return for lighter punishment.The Turkish government insists the country’s judiciary is independent and free from political influence. The case focused on allegations that companies controlled by businessman Aziz Ihsan Aktas secured municipal tenders through bribery, bid-rigging and fraud. The opposition, meanwhile, has said that municipalities under Erdogan’s party control that did business with Aktas were not as thoroughly investigated.

Of the five convicted, the mayor of Istanbul’s Besiktas district, Riza Akpolat, received the heftiest sentence of 23 years and three months for accepting bribes, money laundering and bid-rigging. The sentences of the four other mayors — who like Akpolat were removed from their posts following their arrests — ranged between five and eight years.